This invention relates to steam turbines and more specifically, to the design of last-stage steam turbine buckets with integral covers.
The tip areas of last-stage steam turbine buckets or blades with integral covers operate in a wet steam condition, typically with supersonic relative velocity between the steam flow and the buckets. The action of high speed, wet steam flow on the buckets can produce erosion, and can contribute to corrosion damage of the metal surfaces in the tip areas. The covers between adjacent buckets contact each other during operation by virtue of the bucket's rotation caused by the untwisting effect of the applied centrifugal forces. Connection or contact of the integrally covered buckets during operating conditions enhances the rigidity of the bucket structure and improves vibration damping. The presence of moisture on these contact areas can contribute to stress corrosion cracking. The design of the last stage bucket, therefore, must be tolerant of wet steam in existing environmental conditions. Moreover, any flow disturbing elements at the bucket tip region must be avoided to minimize aerodynamic losses.
The tip bucket design for certain last stage turbine buckets results in a pocket area (or simply, pocket) being formed between adjacent bucket tip covers that tends to trap moisture produced by adjacent surfaces of the bucket covers and leading and trailing edges of the adjacent airfoils. The trapped moisture in the pocket area can cause damage to the buckets themselves as well as the damping contact surfaces of the covers.